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In one of the interviews if I recall Jessica Hecht (Gretchen) mentioned that Walt and Gretchen have been in love and dated. Once it got deeper, Walt got invited for a dinner to Gretchen's wealthy parents' home. Walt started having some kind of inferiority complex afterwards and they eventually broke up.

A: Oh man, he’s a good actor. But it was easy because Vince Gilligan told us exactly what went down between the characters off screen: We were very much in love and we were to get married. And he came home and met my family, and I come from this really successful, wealthy family, and that knocks him on his side. He couldn’t deal with this inferiority he felt — this lack of connection to privilege. It made him terrified, and he literally just left me, and I was devastated. Walt is fighting his way out of going back to that emotional place, so he says, “F— you.”